Seed a new RNG from an old RNG, thereby making the old RNG indepedent of
changing use of the new RNG. The practical effect of this is to reduce the
extent to which changes to child generation algorithm produce a ‘butterfly
effect’ on their parent generators, meaning that generators will be less
likely to produce entirely different outcomes if some detail of a generation
algorithm changes slightly. This is generally good and makes worldgen code
easier to maintain and less liable to breaking changes.